1. Field of the Invention
The invention relates to a method and a feedpump for the metered discharge of a string of a viscous medium.
2. Discussion of the Prior Art
A device for the metered extrusion of a string of a viscous medium is taught by German reference DE 196 47 015 C1. A piston pump is used in this device. The extrusion stroke of the piston of the piston pump is accurately controlled by the piston pump being hydraulically coupled to a gear pump via a high-viscosity hydraulic fluid. The gear pumps used for this purpose are distinguished by a high constancy of delivery and therefore of the extruded string, the start of discharge also being capable of being controlled accurately. However, they are not suitable for the extrusion of abrasive media, for example of sealing compounds enriched with a high percentage of silver flakes and such as are used, for example, for the high-frequency sealing of high-frequency housings.
Seals of this kind are known from German reference DE 197 33 627 C1.
Conventional methods and devices for the metered application of viscous media, so-called dispenser machines, operate on pressure/time-controlled principles. These have the disadvantage that the volumetric flow of the compound to be metered cannot be controlled in proportion to the speed of movement. Since, in most cases, the drives of the coordinate tables moving the metering head accelerate in ramps, uneven metering bead cross sections occur in acceleration and braking phases.
Accordingly, it is an object of the present invention to provide a method and a feedpump for the metered discharge of a string of a viscous medium, in particular of an abrasive medium containing solid particles, which make it possible to have highly accurate application cross sections (beads) at a high speed of movement and, in particular, even in the case of changing speeds, such as occur at radii, etc., of the nozzle discharging the medium.
The basic idea of the invention is to provide a method, in which a constant quantity of the viscous medium is always discharged, irrespective of the speed of movement of the nozzle. This is achieved in that the quantity of the medium is discharged in proportion to the speed of the nozzle and is increased or reduced as a function of the acceleration or deceleration of the nozzle. It is thereby possible always to discharge the same bead cross section of the medium, for example even during the discharge of a curved bead, in which case the nozzle discharging the medium is decelerated, or else during the discharge of a rectilinear bead, in which case the nozzle is accelerated.
In an advantageous embodiment, there is, at the same time, provision for the discharged quantity to be in proportion to the sum, multiplied by a quantity factor, of the nozzle speed, of the nozzle acceleration multiplied by a factor A and of the square of the nozzle speed multiplied by a further factor B:
xe2x80x83Quantity factorxc2x7(nozzle speed+accelerationxc2x7factor A+square of the speedxc2x7factor B).
The discharged quantity can therefore be modified essentially by means of three factors. The quantity factor modifies the total quantity to be discharged and makes it possible to determine whether the bead to be discharged is thicker or thinner. The factor A compensates the dynamic material properties. It takes into account, in particular, whether the viscous quantity behaves in a non-Newtonian way, for example in the same way as a thixotropic medium, in the same way as a dilatant medium or else in the same way as a Newtonian medium. The factor B compensates a speed-dependent nonlinearity which is determined, for example, by a leakage in the pump and by the material behavior (viscosity) of the medium to be discharged.
The constants are preferably determined by measurement as a function of the material to be discharged.
The feedpump according to the invention for the discharge of a string of a viscous medium is particularly suitable for this purpose. The basic idea on which it is based is, on the one hand, to use the accurate metering possibilities of a gearpump and, on the other hand, to make it possible to process even abrasive media. Precisely by means of a feedpump with two gearwheels driven to execute oppositely directed rotation and arranged in a casing so as to be spaced both from the casing walls and from one another, the quantity can be set accurately as a function of the speed of movement and of the movement acceleration or deceleration of the nozzle.
In order, when abrasive media are used, to avoid the pump being damaged due to a xe2x80x9cseizurexe2x80x9d occurring as a result of sintering processes in the particles bound in the medium, a groove spaced equally from the two gearwheels is arranged in each case both in a casing cover and in a casing bottom of the casing. Furthermore, a defined equal spacing between the teeth of the two gearwheels is provided, thus allowing pressure compensation in the region of the teeth, running apart or together, of the gearwheels of the gearpump.
Advantageously, the grooves are in alignment with the inlet and the outlet of the feedpump and extend for a predetermined length in the casing bottom and the casing cover.
In order to allow the gearwheels to run with as little friction as possible, they have, at their upper and lower edge, an essentially circular web-like spacer ring which bears on the casing bottom and the casing cover respectively.
Furthermore, the gearwheels may also have a web-like design in cross section, so that web-like spacer rings can be dispensed with.
The gearwheels preferably consist of one or more of the following materials: plastic, ceramic, metal having a non-stick coating, and metal having a hard anodic coating, the coatings being harder than the abrasive media. In order to avoid contact between the gearwheels and to ensure that a gap is arranged between the gearwheels, they are driven synchronously in opposition to one another by means of an auxiliary gear, in such a way that a gap is formed between the two gearwheels.
The invention is not limited by the embodiments described above which are presented as examples only but can be modified in various ways within the scope of protection defined by the appended patent claims.